1. Field of the Invention
This invention relates to a liquid level sensor, and more particularly to an improved liquid sensor suitable for easily measuring the level variation of an object solution containing electrolyte.
2. Description of the Related Art
Heretofore, a variety of sensors for detecting a liquid level have been developed to measure a stored amount or a flowing amount of liquid.
Typically, such sensors are designed to optically or magnetically detect the position of a float on the liquid surface.
These conventional liquid level sensors are able to detect the liquid level irrespective of the type of liquid but not with high accuracy, and are required to be significantly large-scaled.
If the subject liquid is an electrolyte, a sensor based on the electric conductivity of the solution can be used instead of the float-type sensor.
Such an electrolyte includes those used in the various chemical processes. Further, the sensor of this type can also be applied to measure a sampled amount of body fluids such as urine which have certain electrical conductivity.
FIGS. 7 and 8 show an example of a liquid level sensor for detecting an electrolyte surface level as disclosed in U.S. Pat. No. 5,033,300 "Device for measuring displacement and U.S. Ser. No. 626,616 "Body fluid excretion measurement apparatus for medical application", in which a liquid level sensor 20 is accommodated within a recipient 10 to measure a sampled amount of body fluid of a patient.
The liquid sensor 20 is designed such that a pair of resistance films 24a, 24b are vertically mounted parallel to each other on an insulating substrate 22 made of plastics etc. by printing, vapor deposition or the like.
Such resistance films 24a, 24b typically consist of a carbon resistance, for example, which are printed on the insulating substrate 22 in the shown example.
A plurality of electrodes 26a1, . . . 26a-n, and 26b-1, . . . 26b-m are screen printing vapor-deposited on the resistance films 24a, 24b along their longitudinal direction.
These electrodes 26 are made of e.g. silver thin film disposed so that the electrodes on resistance film 24a are staggered with respect to those on resistance film 24b.
In this embodiment, at the highest stage of the film 24, electrodes 26a-1, 26b-1 are integrated with the lead electrodes 28a, 28b respectively, and coupled to a liquid level circuit through external terminals 30a, 30b.
On the contrary, at the lowest stage, a common electrode 32 is disposed in contact with both of the resistance films 24a, 24b.
Accordingly, when subject liquid is injected into the recipient 10 as shown by an arrow 100, the liquid level 200 changes in accordance with the injected amount, and makes the electrodes 26 mounted on the resistance films 24a, 24b conductive successively. This means that the resistance value between the lead terminals 28a and 28b decreases successively as the liquid level rises, thereby quite easily enabling the detection of the liquid level.
FIG. 9 shows an equivalent circuit of the above-mentioned liquid level sensor. The sensor receives A.C. signals for measurement supplied from an A.C. source 32, and outputs the detected signal to a signal processor 36 through an impedance converting circuit 34 for electrically processing the liquid level data to be displayed thereafter in the calculation/display section 38.
The left and right electrodes 26 are successively and intermittently conducted as the liquid level rises. In this example, the surfaces of the electrodes 26 are coated with a water-repellant layer except at their essential electrode portion for protecting them from being moistened due to the surface tension of the liquid when located above the liquid level 200, at that time.
That is, typically the insulating substrate 22, the resistance film 24, and the electrodes easily get wet. As a result, when the liquid surface is shaken or scattered, the electrodes 26 above the liquid level sometimes undesirably become conductive, thereby causing measuring error.
The water-repellent layer is aimed at preventing such an inconvenience. In FIG. 8, as shown by chain lines, a water-repellent layer 40 is applied to the electrodes 26 with only their center portion being exposed to the liquid. Consequently, at the water-repellent layer 40, the liquid would be nearly all repelled so as not to conduct the electrodes irrespective of the liquid surface shaking or scattering, enabling the elimination of any measuring error.
According to such a type of liquid level sensor 20, the output of the resistance value would become an output changing in stages in accordance with the liquid level for successively conducting the electrodes. Thus, desirable measurement resolution can be obtained by setting the distance between the electrodes properly.
However, in such a liquid level sensor, a disadvantage has arisen; the subject liquid soaks into the resistance films, acting to determine the output resistance, and causes changes in the resistance value resulting in significant measuring error.
Particularly, in the case of carbon resistance or the like having a high water-absorptive property, upon being used once it has been impossible to maintain its original resistance value. In other words, it was not reusable. Further, the resistance film sometimes gets wet even at the first use, causing measuring error.
In the above-mentioned apparatus, the resistance film 24 is coated with the water-repellent layer 40. However, silicon-type coating materials typically used for the water-repellent layer cannot be sufficiently water-proof, causing the resistance film to absorb the liquid through the water-repellent layer 40.
In such a type of liquid sensor, particularly when used for a liquid with the surface gradually varying over a long time, the resistance film comes to absorb the liquid through the portion flooded therein, thereby causing measuring errors.